1. Field of the Invention
The invention relates to an image forming apparatus such as a copying machine, a printer or a facsimile apparatus for effecting image formation by the electrophotographic process or the electrostatic recording process.
2. Description of Related Art
In an image forming apparatus such as a copying machine for effecting image formation by the electrophotographic process, the productivity (the number of prints per unit time) of the apparatus is a very important measure, and in recent years, higher productivity has been required. On the other hand, a higher quality of the output image of the image forming apparatus also is an important measure, and a high quality of image has also been required.
So, in recent years, in order to achieve an improvement in the productivity of the image forming apparatus, design has come to be made so as to transport a transfer material such as paper at a higher speed, but to obtain an output image of high quality, it is particularly important to transport the transfer material such as paper stably to a transferring portion between a photosensitive drum and transferring means (such as a transferring roller).
Particularly when the transfer material is thick paper (cardboard) or the like, if the leading edge of the transfer material (cardboard) is not struck against a predetermined position on the photosensitive drum so that the transfer material fails to be transported to the transferring portion, the transporting speed becomes unstable, and this has sometimes led to a case where the output image is disturbed and the quality of image is reduced particularly when the output image is a halftone image or the like.
So, there is conceived an image forming apparatus in which, as shown in FIG. 16 of the accompanying drawings, a charge eliminating brush member 103, a transport regulating member 104 and an upper transfer guide 105 are installed downstream of a pair of registration rollers 100 provided upstream of a transferring portion (not shown) in the transport direction of a transfer material so that the transfer material can be transported to the transferring portion with the leading edge thereof stably struck against the predetermined position on a photosensitive drum. FIG. 16 shows the background art of the present invention.
The pair of registration rollers 100 are comprised of an upper registration roller 101 and a lower registration roller 102 rotatably brought into pressure contact with each other, and a registration nip portion N is formed between the upper registration roller 101 and the lower registration roller 102.
The pair of registration rollers 100 once stops a transfer material P transported from a feed cassette (not shown) at the registration nip portion N, and are rotatively driven in timed relationship with the formation of a toner image on the photosensitive drum (not shown) to thereby transport the transfer material P to the downstream transferring portion.
The charge eliminating brush member 103 is comprised of a brush portion 103b supported by a brush supporting portion 103a. The transport regulating member 104 has a transport guide portion 104a inclined in the transport direction of the transfer material P. The distal end of the top 104b of the transport guide portion 104a with which the transporting transfer material P contacts is obliquely downwardly bent with respect to the transport direction of the transfer material P. The top 104b of the transport guide portion 104a is located a little above with respect to the horizontal direction in the registration nip portion N.
Downstream of the transport regulating member 104 in the transport direction of the transfer material, there is provided an upper transfer guide 105 for causing the transfer material P being transported to contact with the predetermined position on the photosensitive drum and guiding it to the transferring portion.
The transfer material P nipped and transported by the registration nip portion N between the pair of registration rollers 100 as described above contacts with the brush portion 103b of the charge eliminating brush member 103 and has its charges eliminated thereby, and the leading edge of the charge-eliminated transfer material P strikes against the inclined surface of the transport guide portion 104a of the transport regulating member 104 and is guided to the upper transfer guide 105 while contacting with the top 104b. The upper transfer guide 105 guides the leading edge of the transfer material P so as to strike against the predetermined position on the photosensitive drum. The transfer material P, which has struck against the predetermined position on the photosensitive drum, is transported to the transferring portion between the photosensitive drum and transferring means, and the toner image on the photosensitive drum is transferred to the transfer material P.
By the transport regulating member 104 being provided downstream of the pair of registration rollers 100 as described above, the leading edge of the transfer material P stably contacts with the predetermined position on the photosensitive drum and is directed to the transferring portion. Consequently, the transport of the transfer material P to the transferring portion is stabilized, whereby even if as described above, a halftone image is transferred in the transferring portion by the use of a transfer material such as cardboard disadvantageous to transport, a good output image can be obtained.
Now, in the above-described image forming apparatus, the charge eliminating brush member 103 having the brush portion 103b for eliminating the charges of the transfer material P is provided upstream of the transport regulating member 104, but there has been a case where as the number of prints increases, the brush portion 103b contacting with the transfer material P comes off the brush supporting portion 103a or the brush portion 103b breaks from the surface of the brush supporting portion 103a. 
This is because during the transport of the transfer material P, the transfer material P contacts with the brush portion 103b, whereby the brush portion 103b flexes in the transport direction of the transfer material P, whereby a load is repetitively applied to the root of the brush portion 103b which is exposed from the surface of the brush supporting portion 103a. Further, during the transport of the transfer material P, the transfer material P contacts with the brush portion 103b, whereby the brush portion 103b is sandwiched between the transfer material P and the transport guide portion 104a, whereby the load applied to the brush portion 103b becomes great, and the brush portion 103b becomes liable to come off or break.
As described above, the brush portion 103b has the role of eliminating the charges of the transfer material P upstream of the transferring portion, but will become incapable of appropriately eliminating the charges of the transfer material P before transfer if the brush portion 103b comes off or breaks as described above. If the transfer material P is charged, uneven transfer may occur during the transfer of the toner image in the transferring portion. This uneven transfer appears as a density difference particularly in a halftone image, and is a conspicuous phenomenon.
Also, the brush portion 103b has electrical conductivity and therefore, if the brush portion 103b comes off or breaks, it will scatter in the image forming apparatus, and if the scattered brush portion 103b contacts with an electrical contact portion in the image forming apparatus, electrical leak will be caused and the image forming apparatus may come not to operate normally.
It is an object of the present invention to provide an image forming apparatus in which the coming-off or break of a charge eliminating member by sheet supply endurance can be prevented and the charges of a recording material can be stably eliminated by the charge eliminating member for a long period.
It is another object of the present invention to provide an image forming apparatus having an image bearing member, transferring means for transferring an image formed on the image bearing member to a transfer material, transporting means for transporting the transfer material to between the image bearing member and the transferring means, a regulating member for regulating the transport direction of the transfer material downstream of the transporting means in the transport direction of the transfer material, and a charge eliminating member for eliminating the charges of the transfer material downstream of the transporting means and upstream of the regulating member in the transport direction of the transfer material, the charge eliminating member having a charge eliminating portion and a supporting portion for supporting the charge eliminating portion, the regulating member having a top portion most protruded in a direction from the supporting portion toward the charge eliminating portion of the charge eliminating member, wherein when in a state in which the transfer material is not being transported, the length from the boundary portion between the charge eliminating portion and the supporting portion to the distal end of the charge eliminating portion is defined as xe2x80x9caxe2x80x9d (mm), and the length from the boundary portion to the transport plane of the transfer material in a direction along the charge eliminating portion is defined as xe2x80x9cbxe2x80x9d (mm), and the length from the boundary portion to the top portion of the regulating member is defined as xe2x80x9ccxe2x80x9d (mm), the conditions that axe2x88x92bxe2x89xa70.5 (mm), axe2x88x92bxe2x89xa6b and a less than c are satisfied.
Further objects of the present invention will become apparent from the following description.